calling mount with undef in dir list causes warnings

[jaybuff]

When you specify a user that doesn't exist in the real root (but may exist in the chroot) you get an error:
$ bin/joot foo --user foo
Use of uninitialized value in subroutine entry at /home/jaybuff/joot/lib/Joot.pm line 205.

I believe this is because mount is being passed an undef value in the list of dirs to mount.